Many optical systems utilize optical fibers to transmit light from a remote light source to a target destination. In a typical system, the light source is coupled to the fiber and light supplied by the source is guided by the fiber to the target destination. Optical fibers have been widely used in telecommunications to deliver information encoded in the form of an optical signal. A telecommunication link includes a transmitter that converts an electrical signal to an optical signal. The optical signal is launched into the fiber and transmitted to a receiver that reconverts the optical signal back to an electrical signal for further processing at the destination end of the link. Optical fibers have also been used as point illumination sources. In these applications, light from a source is coupled to the receiving end of the fiber and emerges from the destination end of the fiber as an illuminating beam.
There has recently been interest in extending the use of optical fibers to applications in broad-area illumination. In these systems, the objective is to achieve controlled release of light along at least portions of the length of the fiber. Instead of using the fiber to confine light and transmit it with minimal losses from a source to provide an optical signal or point illumination to a target positioned in the direction of the fiber axis, the objective is to use the lateral surface of the fiber as a broad-area source of illumination that operates in the radial direction of the fiber.
Light-diffusing fibers are a class of fibers that can be used as a broad-area illumination source. Light-diffusing fibers are designed to scatter light propagating in the direction of the fiber axis in radial directions out of the fiber. Radial scattering is typically accomplished by incorporating nanostructural voids in the core region of the fiber. The voids are low-index regions, typically filled with a gas, and have dimensions on the order of the wavelength of the light propagating through the fiber. The refractive index contrast between the voids and surrounding dense glass matrix effects scattering of the light. The scattering efficiency, and hence intensity of scattered light, can be controlled by controlling the dimensions, spatial arrangement and number density of voids. In addition to broad-area illumination, light-diffusing fibers can be employed in displays and as light sources in photochemical applications. Further information about light-diffusing fibers and representative applications can be found in U.S. Pat. Nos. 7,450,806 and 8,591,087, the disclosures of which are hereby incorporated by reference herein
With the increasing trend away from conventional incandescent light sources, LEDs and laser diodes are becoming increasingly important light sources for optical fibers. Efficient coupling of LEDs and laser diodes to optical fibers presents challenges because of mismatches in cross-sectional area and numerical aperture (NA). The cross-sectional areas and numerical apertures of LEDs and laser diodes are much greater than the cross-sectional areas and numerical apertures of typical optical fibers.
One strategy for improving coupling efficiency is to increase the diameter of the optical fiber. The drawback to this approach, however, is that in order to maintain the flexibility of the fiber, it is desirable to maintain the diameter of the glass portion (core+cladding) at or below ˜125 μm. Since much higher diameters are needed for efficient coupling to LEDs and laser diodes, this approach has limited effectiveness in applications where fiber flexibility is desired.
A second strategy for improving coupling efficiency is to increase the numerical aperture of the fiber. This strategy, however, is difficult to implement for conventional light diffusing optical fibers because the voids used in the core region to provide the scattering efficiency required for light diffusing fibers are low index regions and lead to a decrease in the average refractive index of the core. Since high numerical aperture is favored by increasing the refractive index of the core, the need to increase scattering efficiency in light diffusing optical fibers by including low index voids in the core region conflicts with the goal of increasing the numerical aperture of light diffusing fibers and makes it more difficult to achieve efficient coupling between LEDs and laser diodes and light diffusing optical fibers.
There is a need for light diffusing optical elements that couple efficiently to LEDs and laser diodes while maintaining the flexibility needed for deployment in tight space, bent configurations and areas where it is impossible to deploy conventional light sources.